The supply and demand operation of an electric power system is broadly divided into supply and demand planning and supply and demand control. In supply and demand planning, an economic operation plan for a generator for the next day is calculated on the basis of the result of demand forecasting. Supply and demand control is performed so as to cause the output of the generator to follow the demand fluctuation or the like of the day. Supply and demand control includes economical load dispatching control that copes with relatively long-period fluctuations and frequency control that copes with relatively short-period fluctuations. In economical load dispatching control, economic output control of a generator is performed according to a fluctuation. In frequency control, output control of a generator is performed so as to keep the frequency constant.
The role of a secondary batter in the supply and demand planning and the supply and demand control is broadly divided into an improvement in economic efficiency by load leveling and the suppression of frequency fluctuations. Load leveling is optimized at the stage of supply and demand planning. As for supply and demand planning that includes a secondary battery, an electric power generation plan has been proposed which causes a generator with a high power generation efficiency to operate at a higher-efficiency operating point and a generator with a low power generation efficiency to stop (Jpn. Pat. Appln. KOKAI Publication No. 2006-94649 [hereinafter, referred to as “Patent document 1”]). As for the suppression of frequency fluctuations, what suppresses such load fluctuations that the generator cannot follow and output fluctuations of natural energy in frequency control making use of the fast response of the secondary battery, has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 2001-37085 [hereinafter, referred to as “Patent document 2”]).
As for the improvement of economic efficiency in load leveling by a secondary battery, optimization is performed at the stage of supply and demand planning and planned operation is performed at the stage of supply and demand control. Patent document 1 is such that a secondary battery is used for load leveling in supply and demand planning. Patent document 2 is such that a secondary battery is used for frequency fluctuation control in frequency control of supply and demand control.
Since economical load dispatching control has been for handling relatively long-period fluctuations, a second battery has not been used. However, in economical load dispatching control, it has been required to improve economic efficiency more by reducing the fuel cost of the generator. In a small-scale electric power system known as a microgrid which, in recent years, has been discussed actively, the introduction of a secondary battery is indispensable because output fluctuations of natural energy generation or the like have great effects on the system. However, the introduction cost of a secondary battery is high and therefore the effect of introducing a secondary battery has been required to be maximized by more effective use.
Under the circumstances, it is desired to provide techniques for producing greater economic effects with a secondary battery, that is, the effect of decreasing the fuel cost of a generator with a secondary battery, by actively operating a secondary battery from the viewpoint of economic efficiency in economical load dispatching control that handles relatively long-period fluctuations in supply and demand control.